1. Field of the Invention
The present invention relates to a semiconductor device and a soldering method for its manufacture, and particularly to improvement of a thermal stress relaxation member equipped to relax the thermal stress in a semiconductor chip and a method of soldering the semiconductor chip on a prescribed member using the improved thermal stress relaxation member.
2. Description of the Background Art
As known well in semiconductor device manufacturing technology, there are many cases wherein a semiconductor chip formed with desired active regions is soldered on a prescribed member such as an electrode plate. FIG. 6 shows a partial section showing conventional typical soldered structure of a semiconductor chip 9. As shown in FIG. 6, a heat diffusion plate 1, an insulating base plate 3, an electrode plate 5 and a thermal stress relaxation plate 7 are soldered consecutively by using solder layers 2, 4 and 6. The semiconductor chip 9 is soldered on the thermal stress relaxation plate 7 through a solder layer 8. In case the semiconductor chip 9 is a power transistor, for example, the electrode plate 5 corresponds to a collector electrode plate. In addition, the other electrode plates for the semiconductor chip 9, that is, an emitter electrode plate and a base electode plate (not shown) are connected to the semiconductor chip 9 through metallic wires 10.
Among these members, the thermal stress relaxation plate 7 is equipped in order to relax thermal stress generating within the semiconductor chip 9 during soldering process. In case the semiconductor chip 9 is made of silicon and the metallic collector plate 5 is made of copper, since coefficient of thermal expansion of the collector metallic plate 5 is around five times as much as that of the semiconductor chip 9, difference between respective shrinking is great when these are cooled from high temperature conditions in soldering. Accordingly, if the thermal stress relaxation plate 7 is not equipped, the thermal stress corresponding to this difference is directly applied to the semiconductor chip 9 to deteriorate electric characters and strength thereof. Furthermore, the thermal stress relaxation plate 7 is effective for relaxing thermal stress due to heat which is generated during electric current flow through the semiconductor chip 9.
Since the thermal stress relaxation plate 7 is provided for these reasons, the thermal stress relaxation plate 7 must be made of material of which coefficient of thermal expansion is comparatively similar to that of silicon. Besides since heat which generates during operation of the semiconductor chip 9 must be quickly transmitted to the heat diffusion plate 1, large thermal conductivity is necessary for the plate 7. Therefore, a molybdenum flat plate is widely used as a conventional thermal stress relaxation plate 7.
However, the molybdenum flat plate is formed by sintering and is very high in the cost, so that the cost of the semiconductor device equipped with the structure in FIG. 6 is also high.
In addition, as thermal conductivity of the molybdenum is not so large, and efficiency of transmitting to the heat diffusion plate 1 the heat generating during operation of the semiconductor chip 9 is not so high. In other words, in case the molybdenum flat plate is used as the thermal stress relaxation plate 7, it is difficult to reduce transient thermal resistance thereof. This problem turns out to be especially serious upon a transistor for large electric power in which high speed switching operation is repeated.
In order to cope with such a problem, technology using a copper plate embedded with carbon fibers is disclosed in Japanese Patent Laying-Open Gazettes No. 58-32423 (1983) and No. 57-124459 (1982). In this technology, high thermal conductivity is secured by copper and coefficient of thermal expansion is lowered by carbon fibers.
However, in order to suppress thermal expansion of the copper plate, a lot of carbon fibers must be embedded within the copper plate. On the occasion, local warping generates on the thermal stress relaxation plate in high temperature soldering using hard solder. Actually, in the above-indicated Gazette No. 57-124459, it is described that when soldering is carried out above 700.degree. C., the warping of the thermal stress relaxation plate suddenly increases.
In addition, since the thermal conductivity and the electric conductivity of carbon fibers are low as compared with those of metal, if large quantity of carbon fibers is embedded in Cu, the total thermal conductivity and the total electric conductivity of the plate are to too low.